IRAS CPC observations of galaxies: II. Scale-length comparisons and galactic disc opacity

For a study of galactic disc opacity we have measured the disc scalelength at 50 and 100 mu m wavelength of a sample of 55 nearby spirals spatially resolved with the similar to 90'' beam of the IRAS-CPC instrument and compared them to optical scalelengths. The CPC data are independent of the IRAS Survey Instrument data. Reliable far-infrared scalelengths could be determined for 27 galaxies, out to about 3 scalelengths radius; on average. The scalelengths at 100 mu m are on average 1.2 times larger than at 50 mu m, and 1.1 times smaller than in the optical. Two-component dust distribution models were considered for the interpretation, since it is diflicult to match all observational data on far-infrared, visual and near-infrared disc scalelengths with single temperature or single scalelength dust models, especially those with equal intrinsic scalelengths for the stars and the dust. These indicate a total central face-on blue opacity of about 4, and an opacity at the de Vaucouleurs' radius R(25) of less than or similar to 0.05 and similar to 0.9 for the warmer and cooler dust component, respectively, while the latter has a scalelength of about 3 times that of the stars (as measured in the K band) and the warmer dust.